A variety of different polyurethanes are typically prepared by the polymerization of diisocyanates, for example 4,4′-methylenebis(phenyl isocyanate), MDI for short, or tolylene 2,4-diisocyanate, TDI for short, with polyether polyols or polyester polyols. Polyether polyols can be produced, for example, by alkoxylation of polyhydroxy-functional starters. Commonly used starters are, for example, glycols, glycerol, trimethylolpropane, pentaerythritol, sorbitol or sucrose. In the production of polyurethane foams, one of the most important polyurethane systems, additional blowing agents are typically used, examples being pentane, methylene chloride, acetone or carbon dioxide. Water is usually used as chemical blowing agent, which reacts with isocyanate to give polyurea with elimination of carbon dioxide. Typically, the polyurethane foam is stabilized using surface-active substances, especially silicone surfactants.
Polyurethane foams have outstanding mechanical and physical properties and so are used in a very wide variety of fields. The automotive and furniture industries are a particularly important market for various PUR foams (=polyurethane foams), such as conventional flexible foams based on ether and ester polyols, cold-cure foams (frequently also referred to as HR foams), rigid foams, integral foams and microcellular foams and also foams with properties between these classifications, for example semi-rigid systems.
A specific class of polyurethane foams is that of viscoelastic foams. These are also known by the “memory foam” name and are notable both for a low rebound resilience (preferably <10%, whereas the rebound resilience of conventional flexible PUR foams is 35%-45%) and for a slow, gradual recovery after compression (recovery time preferably 2-10 seconds). Materials of this kind are well known in the prior art and are highly valued for their energy- and sound-absorbing properties. Viscoelastic foam materials are encountered in a multitude of fields of use for cushioning (for example, in cushions, seat covers, mattresses, etc.), as sound- and/or vibration-deadening materials, or as impact protection. Typical viscoelastic foams have lower porosity and high density compared to standard flexible ether polyurethane foams. Cushions having a density of usually 30-50 kg/m3 are at the lower end of the density scale typical of viscoelastic foams, whereas mattresses often have a density in the range of 60-130 kg/m3.
Among the viscoelastic foam materials, those made from polyurethanes are of the greatest significance. This is firstly because it is possible, through the choice of the polyol and isocyanate components and of any further auxiliaries used, to very precisely adjust the physical properties of the resultant polyurethane foam, and secondly also because it is possible, through “in situ” production (optionally on site), to produce foam materials of virtually any shape and structure, including very complex shapes and structures.
The majority of the conventional polyurethane foams are block copolymers which have spatially separate regions of different phases having high and low glass transition temperatures (TG). The glass transition temperature divides the brittle energy-elastic region (=glass region) below it from the soft entropy-elastic region above it (=elastomeric region). These high and low glass transition temperatures of different phases within the polymer normally define the temperature range within which the material can be used. The DMA (“dynamic mechanical analysis”) spectra of such materials typically feature a relatively flat region (“modulus plateau”) between the different glass transitions.
The phase of low glass transition temperature in these materials typically (but not always) derives from a “block” of low glass transition temperature, which is preformed and only then subjected to the polymerization. The phase of high glass transition temperature, in contrast, does not normally form until during the polymerization, as a result of the concurrent formation of urethane units. The block of low glass transition temperature (often also referred to as “soft block”) typically derives from a liquid or from an oligomeric resin of low melting temperature, which contains a multitude of groups reactive toward isocyanate units. Polyether polyols and polyester polyols are examples of such oligomeric resins.
In conventional polyurethanes, the hard phases (high glass transition temperature) and soft phases (low glass transition temperature) become arranged with respect to one another during the polymerization and then separate spontaneously from one another in order to form morphologically different phases within the “bulk polymer”. Such materials are also referred to correspondingly as “phase-separated” materials. In this context, viscoelastic polyurethanes are a special case where the above-described phase separation occurs only incompletely, if at all. The glass transition temperature of viscoelastic foams is preferably between −20 and +15° C., but the glass transition temperature of standard flexible polyurethane foams is regularly below −35° C.
Such “structural viscoelasticity” in the case of polyurethane foams having (predominantly) open cells should be distinguished from viscoelasticity attributable to a pneumatic effect. This is because, in the latter case, virtually closed cells are present within the foam material, i.e., only slightly opened cells. As a result of the small size of the orifices, the air flows back in only gradually after compression, which results in slowed recovery. Examples of such viscoelastic foams based on a pneumatic effect are the commercially available products Cosypur® and Elastoflex® from BASF Polyurethanes GmbH.
In general, viscoelastic polyurethane foams are more difficult to produce on a commercial scale than conventional flexible PUR foams. The foaming itself and the curing of the resultant foam are very sensitive to disturbances. Small variations in the composition (for example in the event of variations in the catalyst loading) or in the process regime can lead quickly to reject material. The amount of water in the case of production of viscoelastic PUR foams having typically less than 3 pphp (parts per hundred parts polyol) is smaller than in the case of conventional flexible PUR foams. This fact, in combination with the use of specific polyols, makes it much more difficult to produce viscoelastic PUR foams.
In formulations for production of viscoelastic PUR foams, relative to the amount of water molecules, there are far more polyol hydroxyl groups available for a reaction with isocyanate groups than in formulations for production of conventional flexible PUR foams. The increased competition between polyol and water molecules slows the blowing reaction and hence the formation of CO2. This also results in slowing of the chain extension based on the formation of urea segments. The resulting changes in the blowing and gel reactions frequently lead to unstable structures in the foam or to collapses.
There have already been many descriptions in the prior art of processes for synthesis of polyurethane foams having structural viscoelasticity, these usually having the common feature of the use of a specific polyol composition, in addition to an isocyanate component, which can be chosen more or less freely. Thus, the viscoelastic properties which result from a rise in the glass transition temperature to room temperature are frequently obtained through a combination of various measures during the foaming process. Usually, this specific polyol mixture consists of various polyols. Often, plasticizers are also used.
As well as the above-described problems in the production of viscoelastic PUR foams, it is common knowledge that the cell opening is a particularly critical step in the production of such foams. Particularly when TDI T80 is used as isocyanate component, there is frequently severe shrinkage as a result of insufficient cell opening, which in turn makes it more difficult to run the process.
Furthermore, there is a general trend toward higher foam densities. This is achieved through the use of MDI/TDI mixtures or the use of MDI as the sole isocyanate source. However, higher densities entail a lower water content and hence a lower proportion of chemical blowing agent in the foam formulation. The reduction in the water level results in a smaller amount of urea formed, which in turn results in a foam having lower porosity. In order to compensate for this level of closed cells, large amounts of cell opener are often used in such formulations (for example often more than 3 parts per hundred parts polyol (pphp)). However, large amounts of cell openers frequently have adverse effects on the cell structure and mechanical properties (tensile strength, compression set, expansion, etc.) of the foam.
The polyalkylene oxide cell openers described in WO 2007/146351 are very effective even in small use amounts and are usable in various foam densities and formulations (TDI, MDI or mixtures thereof), but the amount of silicone stabilizer has to be increased considerably for a fine cell structure.
Various further means are known in the prior art for opening of closed cells in flexible polyurethane foams.
For instance, it is possible to open the cells in flexible polyurethane foams by a mechanical route, by flexing the corresponding mouldings after demoulding. This method is commonplace, but is both time-consuming and energy-intensive and is employable only in the production of mouldings.
In addition, the open-cell content of viscoelastic polyurethane foams can be improved by using, as compounds having at least two hydrogen atoms reactive with isocyanate groups, mixtures of at least one polyether alcohol having a high content of ethylene oxide, preferably at least 50% by weight, and at least one polyether alcohol which is incompatible with these polyether alcohols and has a high content of propylene oxide, preferably at least 90% by weight. Thus, U.S. Patent Application Publication No. 2004/0254256 describes viscoelastic foams produced with a polyol component containing 30 to 70 parts by weight of a polyether alcohol having a high proportion of ethylene oxide units in the polyether chain. EP 1 240 228 describes the production of viscoelastic foams using polyether alcohols having an ethylene oxide content in the polyether chain of at least 50% by weight and a hydroxyl number in the range between 40 and 50 mg KOH/g. The use of the ethylene oxide-rich polyether alcohols does increase the open-cell content of the foams, but a disadvantage of the use of polyether alcohols having a high proportion of ethylene oxide in the chain is the increase in the hydrophilicity of the foams. As a result, these foams swell up to 40% by volume on contact with water. This swelling behaviour is unacceptable particularly for applications in moist environments.
Cell openers used in the prior art are still various kinds of additives. For instance, solid particles are frequently used for opening of polyurethane foams. For example, CaCO3 is a standard filler in the PU industry. However, the cell-opening properties of calcium carbonate are usually too low for viscoelastic applications. Very large amounts are required and, even when 15 pphp are used, the result is frequently nevertheless shrinkage of the viscoelastic PUR foams. These high use amounts then additionally lead to an altered cell structure and altered mechanical properties.
A standard method for opening the cells of water-blown polyurethane foams is the use of incompatible liquids, called defoamers. The incompatible liquids, i.e., defoamers, are immiscible with the polyol/isocyanate reaction mixture, but can influence the silicone stabilizers in the reaction mixture at the phase interface and thus facilitate cell opening. Typically, dimethylsiloxanes (silicone oils) are used for this purpose. A known problem with the use of silicone oils is that they promote cell opening at a very early stage of foam formation and hence destabilize the foam. A further disadvantage of the use of silicone oils as cell openers is the very narrow processing window and the usually very coarse cell structure of the resultant foams. In addition, it is also possible to use mineral oils as incompatible liquids. Although these are effective cell openers, mineral oils lead to an oily surface and hence to an unwanted tactile perception of the foams. However, there are also incompatible liquids which are indeed possible cell openers. A problem frequently encountered with incompatible liquids used as cell openers is the unpleasant odour and poor emission characteristics, for example determined in accordance with the test chamber method based on the DIN standard DIN EN ISO 16000-9:2008-04, 24 hours after test chamber loading, as elucidated in detail in the examples section among other places.
A further approach is the use of linear block copolymers consisting of polydimethylsiloxane blocks and polyether blocks [(polyether)(polydimethylsiloxane)]n, as described in U.S. Pat. No. 3,836,560 and DE 10 2006 061 351 A1. These (AB)n structures (i.e., copolymers) are capable of stabilizing the foam in the formation phase by preventing the coalescence of the gas bubbles. At the end of the foam-forming reaction, the (AB)n structures then facilitate cell opening. Below 30 000 g/mol, the (AB)n copolymers have a minor stabilizing effect, and for that reason the minimum average molar mass is typically 30 000 g/mol, preferably between 65 000 g/mol and 100 000 g/mol. However, a problem is that these copolymer structures have a tendency to form hydrogels in the presence of water, which restricts the field of use thereof to a very high degree. A second problem with such (AB)n structures is that the control of the molecular weight during production is not trivial, which leads to a complex process regime. Furthermore, such structures are usually highly viscous because of their high molecular weight, which can lead to difficulties in handling on the part of the processor conducting foaming operations.
In view of the above, there is a need to provide a polyurethane foam (especially a viscoelastic polyurethane foam) having a fine cell structure and a high open cell content.